(Sports Network) - Forgive the Washington Nationals if they arrive in St. Louis a little envious of the Cardinals.

After all, while the Cardinals have already clinched a playoff spot, the Nationals are facing a giant uphill battle to get into the postseason.

Washington looks to keep its fleeting hopes alive on Monday night in the first of three consecutive meetings with St. Louis.

The Nationals split a day-night doubleheader with the Miami Marlins on Sunday. Their loss in the first game actually clinched the Cards' playoff berth and a 5-4 win in the nightcap leaves the Nats five games back of the Cincinnati Reds and Pittsburgh Pirates for the two NL wild card spots.

Washington has six games left to play, three versus St. Louis and three in Arizona.

"I'm not good at math, but I'm good enough to know that losing makes it tougher," Nats third baseman Ryan Zimmerman said. "But you've just got to keep playing and see what happens. We put ourselves in this situation. We've just got to keep trying to finish strong."

Washington's victory in the second game gave Davey Johnson a positive sendoff in his last regular season contest home contest as manager. Johnson plans to retire at season's end, but is convinced the club still has a chance at October baseball.

"We aren't finished. I take nothing for granted," Johnson said. "I still think we've got a good shot."

The Cardinals will be headed to the postseason for a third straight year, but are still gunning for their first NL Central title since 2009, though the club did win the World Series in 2011 as a wild card club.

Still, the Cards would like to avoid the Oct. 1 wild card game and have a two- game edge on the Reds and Pirates for first place in the division. That leaves St. Louis with a magic number of five to wrap up the NL Central title with six home games left to play.

St. Louis, which hosts the Chicago Cubs to close the regular season, was denied a three-game sweep of Milwaukee with Sunday night's 6-4 loss. The Cards committed three errors in the game.

"We made many mistakes and were lucky to have been that close," Cardinals manager Mike Matheny said.

The Cards may need some more luck as they try to hand Tanner Roark the first loss of his Major League career.

Roark has made 12 appearances with the Nationals this season, each of the last three starts. He is 7-0 with a 1.08 earned run average to begin his career and has a 0.95 ERA in his three starts.

All of those have come in August and he hurled his second scoreless outing of the month on Tuesday versus Atlanta. The right-handed Roark contributed seven innings of two-hit ball to the 4-0 win, walking one and striking out six over a career-high 101 pitches.

"What a great game from Roark," said WJohnson. "He was completely in command the entire time he was up there. His pitch count got kind of high there, so that's why I took him out. He earned that win today."

The 26-year-old faces the Cardinals for the first time and goes up against their ace in Adam Wainwright.

The right-handed Wainwright is 17-9 with a 2.98 ERA in 32 starts this year, ranking second in the NL in wins and innings pitched (229 1/3) while tying for second with 209 strikeouts.

Wainwright has given up four runs over his last three outings and three of those came in Wednesday's 4-3 win over Colorado. Wainwright also gave up 10 hits and fanned eight while notching three hits and a pair of RBI at the plate.

The 32-year-old bested Washington on the road on April 23, hurling 8 1/3 scoreless innings with nine strikeouts. He is 5-3 with a 3.09 ERA in this matchup.